Signal-collecting system for radio receivers and the like



Dec. 23, 1941. w. A. SCHAPER 2,267,173

SIGNAL-COLLECTING SYSTEM FOR RADIO RECEIVERS AND THE LIKE Filed Feb. 19,1940 Ww w ATTORNEY *frequency of the signal.

Patented Dec. 23, 1941 PATENT oFncE SIGNAL-COLLECTING SYSTEM FOR RADIORECEIVERS AND THE LIKE William A. Schaper, Cicero, 111., assignor toJohnson Laboratories, Inc., Chicago, 111., a corporation of IllinoisApplication February 19, 1940, Serial No. 319,673

4 Claims.

This invention relates to high-frequency circuits, such as thoseemployed in radio receiving systems. More particularly, the inventionrelates to the portion of such systems which constitutes means forcollecting the high-frequency signals radiated from relatively distanttransmitting, stations. This invention incorporates animproved.signal-collecting means.

Signal-collecting systems generally include a resonant circuit tunableover a desired range of frequencies and classifiable as series orparallel resonant circuits depending upon how the signal voltage isgenerated in or applied to the circuit Collector systems of the seriestype usually em- .ploy a so-called loop antenna or its equivalent tointercept the signals. It is to this type that the present invention isaddressed.

In their practicable forms, series collector systems have heretoforebeen tuned by variation of the circuit capacitance. In the presentapplication, and in my co-pending applications, Serial Numbers 319,671and 319,672, both filed February 19, 1940, to which I shall refer ingreater detail later in this specification, I disclose highlyadvantageous and commercially practicable series collector systems tunedby variation of the circuit inductance, preferably by the employment ofa ferromagnetic core of suitable characteristics movable relatively toan inductance coil in series with the loop or other exposed inductiveelement of the system. This method of tuning has the additionaladvantage of providing means for controlling the high-frequencyresistance of the system in substantially any desired manner as thesystem is tuned over the frequencyrange.

Resonant circuits tuned by inductance variation by means offerromagnetic cores movable relativelyto the inductive element in thecircuit,

and possessing the advantage of simultaneous controlof thecircuit-resistance are disclosed by Polydorofi in United States Re.Patent No.

21,282, in which a resonant circuit having an inductance coil andcapacitor is adjusted over a range of frequencies by movement of acompressed comminuted core relatively to the inductance coil. Thismethod of tuning is called permeability tuning. An improved form of sucha system is disclosed in my United States Patent No. 2,051,012. BothPolydorofis original system and my improved system readily cover anadequate range-of frequencies and may easily be ganged to providemultiple unit systems.

' -In general, the signal voltage generated in a collector system isdirectly proportional to the Thus asignal at the upper end of thebroadcast range will provide approximately three times as much signalvoltage in the collector system as a signal at the lower'end of thisrange. Various expedients have been employed in broadcast receivers inan effort to compensate for this inherent deficiency of the collectorsystem. In my oopending applications above referred to,'and' in thepresent application, I disclose ,collectorisystems in which thisdeficiency is avoided by employing simultaneous variation of circuitinductance and resistance, preferably as provided by permeabilitytuning, in particular circuit arrangements. In accordance withapplication Serial Number 319,672 filed February 19, 1940, I employ anadditional exposed inductive element in an untuned portion of thesystem. In the present application, as I shall later more fullydescribe, I employ an additional unexposed inductive element arranged toregeneratively increase the delivered voltage in a desired mannerthrough a coupling variation also simultaneously produced by therelatively movable ferromagnetic core in a permeability-tuned circuit.

An object of my invention is to provide an improved signal-collectingmeans for radio receivers,

An additional object is to provide a signal-collecting means which maybe successfully employed in the most compact forms of radio receivers,and which may be placed in close proximity to the receiver chassiswithout serious detriment.

Still another object of the invention is to provide asignal-collectingcircuit whose performance characteristics may be readily controlled withrespect to variation with frequency.

It is also an object of the invention to provide a signal-collectingcircuit which may be tuned by inductance variation, for example by meansof a movable ferromagnetic core, and in which the advantages of thismethod of tuning may be realized in accordance with the invention in amanner which will be; readily understood from the following descriptiontaken in connection with the drawingwhich is. a schematic diagram of thebasic circuitarrangement.

Referring to the drawi'mti, ;the improved signal-collecting systemaccording to the present invention comprises an inductive element I, acapacitance 2 and a variable inductance 3 arranged to form a seriesresonant circuit. This circuit is arranged to supply signal voltage tothe grid 4 of vacuum tube 5, which may be the first vacuum tube in aradio receiver, and which may be arranged to act as a high-frequencyamplifier, as a detector or as a modulator in a superheterodyne system.Inductive element I is exposed to the passing radio signals, and maytake the form of a so-called loop antenna. Variable inductor 3 is of thetype in which inductance variation is secured by movement of aferromagnetic element 6, which may be an internal core, relatively tothe winding thereof.

The portion of my improved signal-collecting system just described issimilar to the system described in my copending application, SerialNumber 319,671 filed February 19, 1940, of which the present applicationis a continuation inpart. In thatapplication I- describe in some detailthe performance of the conventional loop circuit tuned by capacitancevariation, and I point out the performance deficiencies therein. I alsopoint out how these deficiencies may at least in great measure beavoided by tuning a series resonant signal-collecting circuit byinductance variation ratherthan capacitance variation."

It will be apparent that since the variably- -tuned circuit includesinductances I and 3, the variable inductor 36 must provide greaterinductance variation than would be required if the inductance I wereabsent, in order to tune the system over the required frequency range.It may be shown that, if L is the effective external series inductance,p is the ratio of the highest to the lowest frequency in the requiredtuning range, and La is the minimum inductance of the variable inductor3-6, then the required effective permeability a of ferromagnetic element6 is Again referring to the drawing, the system according to my presentapplication also includes a winding 1 inductively coupled to variableinductance 3 and connected between the cathode 8 of vacuum tube andground, so thatit is efiectively in the output or plate circuit ofthetube. Winding I acts as a regenerative or feed-back coupling, in aparticular manner later to be described, to materially increase thesignal voltage developed across capacitor 2 particularly at thosefrequencies at which, in the absence of winding 1, the developedvoltages would tend tolbe undesirably low. 5

The introduction of the variable inductor 3-6, with resultant decreasein the permissible inductance of the exposed portion of the collectorcircuit, decreases the voltage generated therein by the signal.'I'hislmight at first be thought to constitute a serious disadvantage.It must be borne in mind, however, that, in the case of a loop, theresonant voltage across the loop at any frequency is not only directlyproportional to the inductance of the loop but also inverselyproportional to the resistance of the loop.

In many of the more compact designs, the loop is placed within arelatively small cabinet and in :close proximityv to the metallicchassisv of the receiver. Under these circumstances the resistance of anormalcondenser-tuned loop is greatly increased, with .resultantdecrease in resonant gain and selectivity. The total. inductance in myimproved circuit may be as great, even at the highest frequency, as in aconventional condenser-tuned loop, and will then be much greater at thelower frequencies. Additionally, I may design the variable inductanceportion of my circuit to have relatively very low resistance, and Ipreferably shield it from the loss-increasing effect of the metallicchassis. Thus the total resistance of my circuit will be materiallylower,

and the inductance as high or higher, than in a conventionalcondenser-tuned loop in the same receiver, with'the result that I thussecure much greater resonant gain and much better selectivity. Theincreased resonant gain more than compensates for the reduced voltagegenerated by the signal in the smaller inductance of the exposed portionof the circuit.

Again referring to the drawing, movement of ferromagnetic core 6relatively to windings 3 and I automatically varies the inductivecoupling between them. In order to effectively compensate for the lowervoltage generated in the inductive element I by the lowerfrequ'encysignals, windings 3 and I are so arranged that the inductive couplingbetween them is materially increased as the ferromagnetic element 6 isadvanced to tune the circuit to the lower frequencies.

As is Well known, the effective high-frequency resistance of apermeability-tuned"circuit increases as the core is advanced into thewinding to tune the circuit to lower frequencies. Delpending upon thematerials employed and the process of producing the core, this increasein resistance may be controlled, and, if: desired, may bemade quitesmall. It is impractical, however, to attempt to entirely compensate forthe lower voltages generated by the signal at the lower frequencies, byemploying a verylow-losscore material and construction. As statedbyJacob in United States Patent No. 2,153,622, a series resonant circuitmay be arranged to produce constant gain by employing a ferromagneticcore which will maintain the ratio of inductive re.- actance toresistance in the circuit, that; is the circuit Q, substantiallyconstant. In carrying out my invention I prefer to employ such a core,and to so arrange the coils 3 and Ithat the'variation in couplingproduced by the movable. core 6 will substantially compensate for thelower voltages generated by the lower frequency signals. Such anarrangement is provided byrthe circuit constants for .a practicalembodiment which I shall later give. It will be understood, however,that since the system provides two performance controls, namely, theresistance variation'and the coupling variation, both providedautomatically by movement of the core, other elections may be madewithin the scope of the invention. For example, if a core is to be em?'ployed which, with the chosen. coil, produces some performancevariation other than that'cor responding to constant-circuit Q, thencoil'l may be so chosen with respect to its winding andpositionrelatively to coil 3 and movingcore 6 as to producea compensatoryvariation of the coupling b'etween coils 3 andl. Additionally, and alsoby virtue of the two controls provided by the invention, any reasonable,desired relation of generated and delivered signal voltages may besecured,

As is well known, by suitable design of the winding 3'and of the'core 6,I may secure any d8:- sirable control of the quality factor Q of theloopcircuit,'that is to say, I am able to regulatedts performance over thefrequency range in any desired manner. 'However, 'the relations differfrom those in a simple inductance-tuned circuit, and control of theoperating characteristics therefore is to be achieved in a somewhatdifferent manner, as I shall now set forth. Let

r l ml 2 v y=ratio of high-frequency to low-frequency resistance of theloop and also of the winding of the variable inductor. z=ratio ofresistance of variable inductor winding to resistance of loop at anyfrequency. =ratio of resistance of variable inductor winding, with thecore inserted so as to tune to any frequency lower than the highestfrequency of the tuning range, to the resistance of the variableinductor winding without the core, both values being measured at saidlower frequency, Neglecting the effect of distributed capacitance, itmay be shown that,

(1) For constant resonant voltage across the circuit with constantsignal strength, the circuit must have constant resistance, and

(2) For constant resonant gain in the circuit, Q=wL/R must be constant,and

(3) For constant selectivity, L/R must be constant, and

2 yp iz- 1 (5) The above relations do not take into account the factthat the distributed capacitance of the variable inductor varies as thecore is inserted. This effect may be minimized in the design of thevariable inductor, and may be compensated for in the development of thecore for a particular design, the above equations serving as a basicguide.

In a successful embodiment of the invention the loop I comprised 17turns of No. 28 plain enameled wire in a rectangular solenoid 5%" by11%", the winding being long, and having an inductance of 155 ,uh. Thevariable inductor coil 3 comprised a progressive universal winding ofsingle silk enameled litz wire, 1.24" long on a tube of 0.2" insidediameter, 0.007" wall, the inductance being 246 h.

Ferromagnetic element 6 may comprise a compressed comminuted core ofhydrogen-reduced powdered iron that has been sifted through a screenhaving 400 meshes to the inch. For use with the above describedinductor, it may be 0.200" in diameter and 1 long, being preferablyhot-molded at 180 F. with 0.5% particle insulation and 3% of powderedBakelite binder, and cured at 290 F. for 3 hours. Such a core will havean effective permeability of about 11.5.

Winding I comprised 2 /2 turns of No. 36 plain enameled wire wound to alength of on a tube of 0.325 outside diameter and mounted concentricallyover the right-hand or grid end of coil 3, As indicated in the drawing,ferromagnetic core 6 was arranged to enter the left-hand or loop end ofcoil 3. It will be apparent that by other dispositions of winding 1, orby arranging to have core 6 enter the opposite end of coil 3, or both,the variation of the inductive feedback coupling may be altered.

In the illustrative embodiment just described,

resistor 9 was of 500 ohms and capacitor It! was of 0.1 microfarad.Capacitor 2 was adjusted to tune the circuit to the highest frequency inthe range with core 6 fully removed from coil 3. Vacuum tube 5 was ofthekind known to the trade as the type 1232. The frequency range of thesystem was from 540 kc. to 1500 kc.

It will be understood that the above constants are given merely by wayof illustrative example and are therefore not to be taken as in any walimiting the scope of my invention.

My improved signal-collecting system in-. cludes a closed seriesresonant circuit having a fixed or adjustable capacitor, an inductiveportion exposed to the signals, which may be a loop, and anunexposedinductive winding. The system is tuned to resonance with anydesired signal within a wide range of frequencies solely by variation ofthe inductance of the winding, produced by means of a ferromagnetic coremovable relatively to the winding. My closed series resonant circuit isto be distinguished from open series resonant circuits in which part ofthe series capacitance is the capacitance of an open-ended antenna toground. The exposed inductive portion of my closed series resonantcircuit has capacitance to ground, but this capacitance is effectivelyin parallel to the exposed inductive portion of the circuit.Additionally, my improved signal-collecting system provides means forproducing a delivered signal voltage which is substantially independentof the frequency of the desired signal, these means including a vacuumtube and an inductive feedback coupling which is automatically varied ina desired manner by the tuning means.

In the illustrative embodiment above described, the coupling betweencoil 3 and winding 1 was well below the value which, with the tubeemployed, would correspond to incipient self-oscillation of the system,at all frequencies within the range. Couplings which will not produceselfoscillation should invariably be employed for obvious reasons.

Having thus described my invention, what I claim is:

1. A signal-collecting system for use in radio receivers and the likehaving a first vacuum tube, including a closed series resonant circuithaving a capacitor, an exposed inductive portion and an unexposedinductive winding connected to an input terminal of said vacuum tube, acoil connected in the output circuit of said vacuum tube and positionedadjacent one end of said winding, and a ferromagnetic element movablyarranged to enter the other end of said winding to tune said circuitover a, range of frequencies and to simultaneously vary the couplingbetween said winding and said coil, whereby the signal voltage deliveredto saidvacuum tube is substantially independent of the frequency of thesignal.

'2. A signal-collecting system for use in radio receivers and the likehaving a first vacuum tube with control electrode, cathode and plate,including a closed series resonant circuit comprising a capacitor and anexposed inductive portion having a common ground connection and anunexposed inductive winding having a terminal connected to said controlelectrode, a coil connected between said cathode and ground andinductively coupled to said winding and positioned adjacent saidterminal, and a ferromagnetic element movably arranged to enter the endof said winding remote from said terminal to tune said circuit over arange of frequencies and to simultaneously vary the coupling betweensaid winding and said coil, whereby the signal voltage delivered to saidvacuum tube is substantially independent of the frequency of the signal.

3. A signal-collecting system for use in radio receivers and the likeincluding a closed series resonant circuit having a capacitor, anexposed inductive portion and an unexposed inductive winding, aferromagnetic element movably arranged to enter one end of said windingto tune said circuit over a range of frequencies and adapted to maintainthe reactance-to-resistance ratio of said winding substantiallyconstant, and regenerative means including a coil positioned adjacentthe other end of said winding, whereby its eifect is automaticallyregulated in approximate inverse proportion to the frequency of thesignal by motion of said element.

4. A signal-collecting system for use in radio receivers and the likeincluding a first vacuum tube having a control electrode, a cathode anda plate; a closed series resonant circuit comprising a capacit'ora'nd anexposed inductive portion having a common ground connection and anunexposed inductive winding having a terminal connected to said controlelectrode; a ferromagnetic element movably arranged to enter one end ofsaid winding and adapted to maintain the reactance-to-resistance ratioof said circuit substantially constant while tuning said circuit over arange of frequencies; and a regenerative coil connected between saidcathode and ground and positioned adjacent the other end of saidwinding, whereby its effect is automatically regulated in approximateinverse proportion to the frequency of the signal by motion of saidelement.

WILLIAM A. SCHAPER.

